Nature exhibits highly symmetrical polyhedral cage-like structures at different scales, such as viral capsids, clatherin-coated vesicles, and the protein transport complex, COPII. All are connected mathematically by Archimedean polyhedrons and from a synthetic chemistry perspective, by the common thread of protein complex self-assembly. Inspired by both biomolecules and their underlying geometric principles, chemists have developed routes to create capsule and polyhedral assembles utilizing hydrogen-bonding or coordination interactions. Replicating large, multi-component, three-dimensional architectures in vitro is synthetically challenging, due in part, to the precise control necessary for building block assembly and the fact that subtle changes in the subunits can give very different results. Notable achievements toward the synthetic construction of very large, cage-like constructs include cuboctahedrons and dodecahedrons (5.2 and 7.5 nm, respectively, based on pulsed gradient spin-echo NMR), rhombicuboctahedrons and sphere-in-sphere rhombicuboctahedrons (5.0 and 6.3 nm, respectively, based on synchrotron X-ray crystal data).
The eloquent architecture of the protein complex COPII and its structural connection to Archimedean polyhedrons provided inspiration for the design and construction of highly symmetric, 3D, supra-molecular structures. The molecular motif and critical parameters of building block preciseness-of-fit and method of connectivity were considered. Ligand-metal-ligand building block connectivity provides desirable synthetic characteristics by facilitating metal coordination sites to act as either vertices or as edges in a contemplated shape. The linearly-coordinated, pseudo-octahedral, <tpy-M2+-tpy> complex (where, tpy=[2,2′:6′,2″]terpyridine) has been demonstrated to be a good option for the fabrication of 2D and 3D supramolecules.
FIG. 1 illustrates the construction of simple and complex 2D polygons including a Sierpinski triangle and gasket, based on single-angle components and derivation of Archimedean polyhedrons using multi-angle, building blocks possessing increasingly greater angles (β), where the critical dihedral angles within the cuboctahedron are 125° corresponding to β=90°.
Use of [2,2′:6′,2″]terpyridine, as a readily available and easily functionalized monomer, has been reported for the self-assembly of numerous 2D, metallomacrocycles including, triangles, hexagons, a Sierpinski gasket, and a Sierpinski triangle, wherein the vertices were connected through terpyridine-based coordination chemistry.
Precise control over the interconversion between different supramolecular assemblies remains a desirable goal.